Eye Development
The major development of the eye takes place between week 3 and week 10 and involves ectoderm, neural crest cells, and mesenchyme. The neural tube ectoderm gives rise to the retina, the iris and ciliary body epithelia, the optic nerve, the smooth muscles of the iris, and some of the vitreous humor. Surface ectoderm gives rise to the lens, the conjunctival and corneal epithelia, the eyelids, and the lacrimal apparatus. The remaining ocular structures form from mesenchyme.
Development of the Optic Cup and Lens Vesicle
On or about day 22, two small grooves develop on each side of the developing forebrain in the neural folds. They are called optic grooves, or optic. As the neural tube closes, these grooves become outpocketings and are now called optic vesicles. The optic vesicles extend from the forebrain toward the surface ectoderm through the adjacent mesenchyme. As the optic vesicles grow toward the ectoderm, their connections to the forebrain become attenuated to form optic stalks, which will eventually become the optic nerves.
The portion of each optic vesicle that interacts with the surface ectoderm induces that area of the ectoderm to form a thickening called the lens placode (a precursor of the lens). The lens placode invaginates to become a lens pit, which soon forms a complete circle that pinches off from the surface ectoderm to become the lens vesicle. At the same time the lens vesicle is forming, the optic vesicle also invaginates to form a double-layered structure called the optic cup. So, at this point we see a goblet-shaped optic cup with the lens vesicle floating in its open end.
The developing optic vesicle and stalk have a groove on their inferior surfaces called the optic, or choroidal, fissure, through which blood vessels gain access to the optic cup as well as the lens vesicle. The blood vessels are the hyaloid artery, a branch of the ophthalmic artery, and its accompanying vein. The choroidal fissure will eventually fuse, completing the eye wall inferiorly and enclosing the vessels in a canal in the optic stalk. When the lens matures later on in fetal life, the distal end of the hyaloid artery will disintegrate and its proximal end will persist as the central retinal artery.
The two layers of the optic cup will further differentiate into the retina of the mature eye.
The two layers are unequal in size - the outer one is thinner than the inner one. The optic cup can be divided into two portions, the anterior 1/5 (rim) and the posterior 4/5. The rim area will ultimately form the iris and ciliary body, and the posterior 4/5 will form the retina (Figure 1). The outer layer of the posterior 4/5 will become the pigment layer of the retina, and the inner one will become the neural retina. These two layers are separated by the intraretinal space.
The development of the retina’s pigment layer is very straightforward, with the appearance of melanin granules in the cells of this layer at around 4 1/2 weeks. Slightly later, at about 6 weeks, the cells in the posterior aspect of the inner layer of the optic cup begin a more complicated process. The cells immediately adjacent to the intraretinal space begin to differentiate into the photoreceptors (rods and cones). The next layer of cells will become the Muller supporting cells and the bipolar neurons, and the innermost superficial layer will develop into the axons of the ganglion cells, the ones that will make up the optic nerve. This means light actually passes through the neuronal layers before reaching the rods and cones. The ganglion cell fibers gradually fill in the lumen of the optic stalk as it becomes the optic nerve. By eight months, all the layers of the retina that you will see in your Histology course are recognizable. But maturation of the photoreceptors continues after birth, which in part explains why a baby’s visual acuity improves as he or she grows.
At about the same time as the pigmented layer of the retina is developing, the cells of the posterior part of the lens vesicle transform into elongated, slender primary lens fibers. These new cells fill in the previously hollow structure. About four weeks later, more lens fibers develop, this time from the anterior wall of the lens vesicle (secondary lens fibers).
Development of the Choroid, Sclera and Cornea
During the sixth and seventh weeks the mesenchyme that surrounds the external surface of the optic cup condenses into two layers, an inner, pigmented, vascular layer known as the choroid and an outer, fibrous layer called the sclera. The mesenchyme that is anterior to the developing lens splits into two layers that surround the newly formed anterior chamber of the eye. The inner layer is continuous with the choroid and is called the iridopupillary membrane and the outer layer is continuous with the sclera. The outer layer will form the substantia propria, or stroma of the cornea. The cornea has three layers, epithelium, stroma, and endothelium. The external corneal epithelium develops from surface ectoderm and the endothelium forms from neural crest cells that migrate from the rim of the optic cup. As mentioned above, the stroma is derived from the surrounding mesenchyme. The iridopupillary membrane eventually disappears completely, which allows communication between the anterior and posterior eye chambers.
Development of the Iris and Ciliary Body
The anterior rim of the optic cup gives rise to the epithelium of the iris and the ciliary body. Remember that the inner layer of the posterior 4/5 of the optic cup forms the neural retina of the eye. The anterior part of this inner layer forms the non-pigmented layer of the iris and the ciliary process epithelium. The outer layer of the optic cup in this region contributes the pigmented epithelial layer. A few folds form in the anterior aspect of the optic cup and this forms the ciliary processes. The stroma of the iris and the ciliary body develop from neural crest cells that migrate into the area. Within the stroma of the iris, the sphincter pupillae and dilator pupillae muscles develop from optic cup neuroectoderm. In contrast, the ciliary muscle, which is responsible for changing the shape of the lens, is derived from overlying mesenchyme. The color of the eye is determined by the amount of melanin distributed in the stroma of the iris. Eyes of all colors have melanin in the epthelium on the posterior aspect of the iris.
The vitreous body forms in the center of the optic cup posterior to the lens. It is comprised of a gel-like substance called vitreous humor derived from mesenchymal cells of neural crest origin. Later on more vitreous humor is added which is believed to come from the neuroectoderm of the optic cup.
The eyelids begin to form in the sixth week from neural crest cells as well as surface ectoderm just anterior to the cornea. They begin as two folds of skin that meet over the cornea and they are attached to one another until about the 27th week when they separate. While they are adherent to one another there is a conjunctival sac between the eyelids and the cornea. The orbicularis oculi, which is found within the eyelids, forms from the second branchial arch along with the other muscles of facial expression and will be innervated with SVE fibers.
The extraocular muscles develop from three preotic somites. These are the somites founds anterior to the developing ear of the embryo. Each preotic somite is supplied by its own cranial nerve. Remember three different cranial nerves (III, IV, and VI) supply the extraocular muscles. So the somite which is supplied by the III cranial nerve forms 5 of the 7 extraocular muscles while the remaining two each give rise to one muscle each.
Regulation of Eye Development
Normal development of the eye requires a rather complex interplay between different tissues of the eye and involves several reciprocal inductive events (Fig. 2). The PAX6 gene product, a transcription factor, is a key player in the process. Development of the eye begins with the designation of a single eye field in the neural plate before neurulation begins. The separation of this single eyefield into two eyefields is dependent upon secretion of sonic hedgehog (shh) from the prechordal plate. It has been suggested that the sonic hedgehog protein suppresses the expression of the PAX6 gene and upregulates the PAX2 gene in the anterior neural ridge, which causes the field to divide in two. A defect in the sonic hedgehog protein or its expression, therefore, results in cyclopia.
In the third week when the optic vesicle buds from the neuroectoderm, it induces the overlying surface ectoderm to form the lens placode by secreting the growth factor BMP4. The ability of the surface ectoderm to respond to BMP4 is dependent on the expression of the PAX6 gene in the surface ectoderm. The lens placode in turn becomes the inducer and secretes growth factors (FGF among them) that induce the optic vesicle to differentiate into the optic cup. Then as the lens vesicle forms from the lens placode it secretes factors that induce the formation of the neural retina in the wall of the optic cup. In addition, the lens vesicle also induces the overlying ectoderm to begin forming the cornea. Now the neural retina becomes the inducer and secretes factors that cause the cells at the inner aspect of the lens vesicle to elongate and become lens fibers. As the inner neural retinal layer is forming, the mesenchyme surrounding the optic cup secretes transforming growth factor (TGF), which induces the formation of the pigmented retinal layer as well as the choroid and the sclera.
Steps in infant vision development
At birth, babies can't see as well as older children or adults. Their eyes and visual system aren't fully developed. But significant improvement occurs during the first few months of life. The following are some milestones to watch for in vision and child development. It is important to remember that not every child is the same and some may reach certain milestones at different ages.
Birth to 4 months
At birth, babies' vision is abuzz with all kinds of visual stimulation. While they may look intently at a highly contrasted target, babies have not yet developed the ability to easily tell the difference between two targets or move their eyes between the two images. Their primary focus is on objects 8 to 10 inches from their face or the distance to the parent's face.
During the first months of life, the eyes start working together and vision rapidly improves. Eye-hand coordination begins to develop as the infant starts tracking moving objects with his or her eyes and reaching for them. By eight weeks, babies begin to more easily focus their eyes on the faces of a parent or other person near them.
For the first two months of life, an infant's eyes are not well coordinated and may appear to wander or to be crossed. This is usually normal. However, if an eye appears to turn in or out constantly, an evaluation is warranted.
Babies should begin to follow moving objects with their eyes and reach for things at around three months of age.
5 to 8 months
During these months, control of eye movements and eye-body coordination skills continue to improve.
Depth perception, which is the ability to judge if objects are nearer or farther away than other objects, is not present at birth. It is not until around the fifth month that the eyes are capable of working together to form a three-dimensional view of the world and begin to see in-depth.
Although an infant's color vision is not as sensitive as an adult's, it is generally believed that babies have good color vision by 5 months of age.
Most babies start crawling at about 8 months old, which helps further develop eye-hand-foot-body coordination. Early walkers who did minimal crawling may not learn to use their eyes together as well as babies who crawl a lot.
9 to 12 months
At around 9 months of age, babies begin to pull themselves up to a standing position. By 10 months of age, a baby should be able to grasp objects with thumb and forefinger.
By twelve months of age, most babies will be crawling and trying to walk. Parents should encourage crawling rather than early walking to help the child develop better eye-hand coordination. • Babies can now judge distances fairly well and throw things with precision.
1 to 2 years
By 2 years of age, a child's eye-hand coordination and depth perception should be well developed.
Children this age are highly interested in exploring their environment and in looking and listening. They recognize familiar objects and pictures in books and can scribble with crayons or pencils.